In like manner to mixtures of two or more metals or nonmetals being termed metal alloys and nonmetal alloys, respectively, mixtures of two or more metal oxides yielding solid solutions have frequently been described as ceramic alloys. One well known example of the latter is stabilized and partially-stabilized ZrO.sub.2.
With respect to microstructure, two principal types of partially-stabilized, polycrystalline ZrO.sub.2 bodies have been discussed in the prior art. The first type is composed of relatively large size (.about.30-100 micron) cubic ZrO.sub.2 grains, customarily including a dopant, containing precipitates of ZrO.sub.2 having a tetragonal structure of submicron size with lower dopant content therewithin, and precipitates of ZrO.sub.2 having a monoclinic and/or tetragonal structure of about 1-10 microns in size along the grain boundaries of the cubic grains, thereby yielding a microstructure consisting of a ceramic alloy comprising ZrO.sub.2 and one or more stabilizing agents, with MgO and/or CaO and/or Y.sub.2 O.sub.3 being the most commonly used of such agents. The second type of partially-stabilized, polycrystalline ZrO.sub.2 bodies is composed of a ceramic alloy of ZrO.sub.2 with one or more stabilizing agents of submicron grain size and most typically having a tetragonal structure. This second type of ceramic alloy is frequently a combination of ZrO.sub.2 and Y.sub.2 O.sub.3, although CeO.sub.2 has been used, as could combinations of Y.sub.2 O.sub.3 with MgO and/or CaO. In principle, any number of the rare earth metal oxides would also be operable for alloying.
ZrO.sub.2 -containing ceramic bodies exhibiting the highest toughness values have involved MgO stabilization to yield the large grain size microstructure described above. Such bodies have demonstrated toughness values in excess of 10 MPa.sqroot.m when measured by short beam, chevron notched beam, and single edge notched beam types of K.sub.IC tests. In contrast, laboratory testing has shown that the toughness values of the fine-grained ZrO.sub.2 ceramic alloys discussed above are substantially lower. For example, a fine-grained body of ZrO.sub.2 with 3 mole percent Y.sub.2 O.sub.3 manifests a K.sub.IC of about 6 MPa.sqroot.m, and fine-grained ZrO.sub.2 bodies stabilized with 2 mole percent Y.sub.2 O.sub.3 gave values no higher than about 10 MPa.sqroot.m.
This difference in fracture toughness can be explained on the basis of microstructure. Thus, as was described above, the large grain size bodies not only contain precipitates of submicron size having a tetragonal structure, but also contain precipitates of larger size (.about.1-10 microns) having a tetragonal and/or monoclinic structure along the grain boundaries. The tetragonal structure of the larger-grained precipitates can transform to the monoclinic polymorph during cooling from the fabrication temperature of the body, or can transform in the stress field set up by a propagating crack. Those relatively large-dimensioned precipitates along the grain boundaries of the cubic ZrO.sub.2 that have been transformed into the monoclinic structure are areas of high stress which can act to promote the transformation of the smaller-sized precipitates of tetragonal structure within the cubic grains to the monoclinic form. Optical and electron microscopy examinations have indicated the presence of a large zone, i.e., .about.100 microns, of transformed precipitates (both large and small grain) around propagating cracks. It is that large zone of transformation which gives rise to the high toughness.
In contrast, the microstructures of the second type of partially-stabilized ZrO.sub.2 bodies are quite uniform with grain sizes generally varying between about 0.3-1 micron, depending upon firing temperature, time at sintering temperature, particle size of the starting batch materials, and composition. There are no fine-grained precipitates formed as each tetragonal grain acts as a single precipitate. There is a lack of development of the large size precipitates along the grain boundaries. As a result, the transformation zone around propagating cracks in this type of body is only on the order of 4 microns or less. It has been mathematically predicted that the level of toughening should be directly proportional to the square root of the dimensions of the transformation zone. The measure of toughness reported above on the two types of bodies qualitatively confirms that prediction.